There's a new window stuff like rain off called dux, ducx or something similar.
I do the windows of a Halfords Auto Centre and they can put this stuff on the car windscreen for customers. It supposed to last a year. They done one of the show room windows with it. When I do the windows, it looks just as dirty as all the others.
It's about £30 for a small sachet which is enough to do one windscreen.